A silver halide photographic material comprises a silver halide emulsion layer provided on a support. The silver halide emulsion is usually chemically sensitized to obtain a desired sensitivity or gradation. Examples of the chemical sensitizer include a sulfur sensitizer, a selenium sensitizer, a tellurium sensitizer, a noble metal (such as gold) sensitizer, a reduction sensitizer and a combination thereof. The sulfur sensitizer has been most frequently used in a conventional photography. The sulfur sensitizer is a labile sulfur compound which reacts with silver ion to form silver sulfide. The sulfur sensitizer is described in P. Grafkides, Chimie et Physique Photographique (Paul Montel, 1987, fifth edition); T. H. James, The Theory of The Photographic Process (Macmillan, 1977, fourth edition); and H. Frieser, Die Grundlagender Photographischen Prozess mit Silver-halogeniden (Akademische Verlagsgeselbshaft, 1968).
Examples of the conventional sulfur sensitizers include thiosulfates (e.g., sodium thiosulfate); thiosulfonates (e.g., p-toluenethiosulfonate); thioureas (e.g., allylthiourea, N,N'-diphenylthiourea, triethylthiourea, acethylthiourea, N-ethyl-N'-(4-methyl-2-thiazolyl)thiourea); thioamides (e.g., thioacetamide, N-phenylthioacetamide); rhodanines (e.g., rhodanine, N-ethylrhodanine, 5-benzylidenerhodanine, 5-benzylidene-N-ethylrhodanine); thiohydantoins; 4-oxo-oxazolidine-2-thiones; dipolysulfides; thiosulfonic acids; mercapto compounds (e.g., cysteine); polythionates; sulfur of simple body; and sodium sulfide.
Further, U.S. Pat. No. 4,115,129 (Bigelow) discloses an organophosphine sulfide as a sulfur sensitizer. The sensitizer should be used at a pH of at least 7. The organophosphine sulfide is represented by the formula: ##STR2## wherein
(A) R is individually selected from hydrogen and a monovalent organic radical; m is 0, 1 or 2, and not more than one of said R's is hydrogen when m is 0; and X represents a group having the formula: ##STR3## where Z is a divalent organic radical, and n is 0 or 1, or (B) (x).sub.m and (R).sub.3-m together comprises a single trivalent heterocyclic radical.
Examples (31) of the organophosphine sulfide are disclosed at columns 4 and 5 in U.S. Pat. No. 4,115,129. Most of the examples (22) are relatively large molecular compounds having 13 or more carbon atoms. The other nine examples having 12 or less carbon atom are shown below. ##STR4##
The organophosphine sulfides disclosed in U.S. Pat. No. 4,115,129 have also been known as supersensitizers, as is disclosed in U.S. Pat. No. 3,895,951 (Riester et al.).
By the way, the chemical sensitization is greatly influenced by external conditions such as pH, pAg, temperature and time. Accordingly, the conditions are preferably kept constant to obtain a good reproducibility of photographic products. However, it is practically difficult to obtain a constant value of pH or pAg in preparation of silver halide emulsions. Therefore, a sulfur sensitizer is required to sensitize a silver halide emulsion constantly, even if the pH and pAg conditions are changed. The above-mentioned conventional sulfur sensitizers do not satisfy the requirement. For example, as is shown in Example 1 (cf., Table 1) of the present specification, thioureas, thioamides and rhodanines are unstable with respect to the sensitivity of a silver halide emulsion when the pH of the emulsion is changed, while thiosulfates are relatively stable. On the other hand, thiosulfates, thioureas and thioamides (particularly thiosulfates) are unstable when the pAg of the emulsion is changed, while rhodanines are relatively stable, as is shown in Example 2 (cf., Table 2). It might be concluded from these results that a small molecular sulfur sensitizer (e.g., thiosulfates) is unstable when the pAg of the emulsion is changed, while a relatively large molecular sulfur sensitizer (e.g., rhodanines) is unstable when the pH of the emulsion is changed.
Examples of the organophosphine sulfide disclosed in U.S. Pat. No. 4,115,129 are unstable when the pH of the emulsion is changed, as is shown in Example 4 (cf., Table 4) in the present specification. Accordingly, U.S. Pat. No. 4,115,129 describes that the sensitizer should be used at a pH of at least 7.